1. Field of the Invention
This invention relates to improvements in current source circuits, and more particularly to improvements in current source circuits that provide a current that is proportional to absolute temperature (PTAT). 2. Relevant Background Information
Current source circuits that provide a current that is proportional to absolute temperature have many uses. In the past, such circuits suffered numerous deficiencies. Cascode current mirrors having a high output impedance, were necessary to force equal currents in two bipolar transistors of different emitter areas, thereby increasing the minimum operating supply. Also, reliable start-up was not enjoyed under all conditions. When start-up circuitry was present, it had to be disconnected from the circuit not to affect the output current value when an equilibrium was reached. Sensing of that equilibrium was also difficult to implement in a reliable way. Other shortcomings of the classical solutions were the use of operational amplifiers, or transistor current gain dependent output.
A typical PTAT current source 10, in accordance with the prior art, is shown in FIG. 1. The prior art circuit for generating a PTAT current has two complementary current mirrors, as shown in FIG. 1. The current source 10 includes a first current mirror that is provided by NPN bipolar transistors 11 and 12 and a current mirror provided by a P-channel MOS transistors 17 and 18. The two NPN transistors 11 and 12 connected as shown to provide respective current paths from a V.sub.CC rail 15 to ground 16 through respective MOS transistors 17 and 18. The bases of the NPN transistors 11 and 12 are interconnected to each other and to the collector of the NPN transistor 12. The emitters of the NPN transistors 11 and 12 are sized such that the emitter of the NPN transistor 11 is n-times larger than the emitter of the NPN transistor 12. A resistor 20, of value R, is connected between the emitter of the NPN transistor 11 and the ground rail 16. A current source 22 that provides a current of magnitude Istart is connected between the gates of the MOS transistors 17 and 18 and the ground rail 16. The gates of the MOS transistors 17 and 18 are interconnected with each other and to the drain of the MOS transistor 17. An output MOS mirror transistor 23 is connected to provide a current path from the V.sub.CC rail to an output terminal 24 from which the output current Iout is derived. The gate of the MOS transistor 23 is connected to the gates of the MOS transistors 17 and 18, whereby the output current Iout that is delivered to terminal 24 mirrors the current that flows through the MOS transistor 18 and NPN bipolar transistor 12.
If the mirrors are ideal, the collector currents of NPN transistors 11 and 12, I.sub.c11 and I.sub.c12, are equal. Thus: ##EQU1## or, taking the logs: EQU V.sub.be12 V.sub.be11 +V.sub.t .multidot.log (n).
Therefore, the voltage drop across the resistor 20, of value R, which has the value V.sub.be12 -V.sub.be11, equals: EQU RIout=V.sub.t .multidot.log (n)
With ideal mirrors, one should then obtain for the output current: ##EQU2##
Practical implementations of the circuit described, however, suffer from the non-ideal state-of-the-art current mirrors that result mainly from the Early effect on their outputs. Reducing this effect requires cascoding the mirrors, which then will not operate under low supply voltages. With respect to the lower mirror that includes the bipolar transistors 11 and 12, base current effects also need to be eliminated. The sources of error contribute to make the collector currents in the transistors 11 and 12, I.sub.c11 and I.sub.c12, different, and then the ideal PTAT relationship for Iout becomes inexact.
Another problem is the condition where the circuit 10 reaches its second equilibrium state at the moment of startup, corresponding to zero value collector currents in the transistors 11 and 12, i.e., I.sub.c11 =I.sub.c12 =0. Since this equilibrium is also stable, it is generally avoided by adding the startup current source, Istart, 22 into the input of one of the mirrors to initiate current growth in the transistors at power up. Since this current source also affects the current output value when the equilibrium is reached, it needs to be disconnected from the mirrors at that time by an adequate detection circuitry that senses when the output has become stable. Such a detection circuit needs to disconnect the current source 22 precisely after the critical threshold of ##EQU3## has been crossed, not to abort the current growth in the mirrors before that point. It therefore requires a current source similar to the PTAT current we are considering, and would also make this circuit prone to oscillation, since the current in the mirrors also tends to recess and to drop back below the threshold when the current from the current source 22 is disabled too quickly.
In general, PTAT current sources suffer from either poor accuracy or uncertain startup behavior. If startup circuitry is proposed, it often affects the value of the output current, especially for large startup current values. Other circuits require the use of operational amplifiers, which are more costly in silicon area.